The present invention relates to a container for reheating frozen food products in a microwave oven. The invention is particularly useful for reheating large size frozen meals that typically require excessively long heating times.
The long length of time required to reheat large size frozen meals in a microwave oven is a real concern in the food service and catering business. For individual portions or small size frozen meals, reheating in a domestic microwave oven, can be carried out in a relatively short period of time, generally in the range of 2 to 6 minutes, depending on such factors as, for example, the type of foods, the size of the food components, and the lay-out of the various food components in the tray. For large size frozen meals, however, microwave reheating has proven to be excessively long, for example, up to 30 minutes. These long reheating times for large frozen meals renders the use of microwave ovens less attractive.
Another problem with re-heating frozen products in a microwave oven is that temperature gradients occur in the food when it is reheated in most known containers. Before the food product is thawed, the frozen product is essentially transparent to microwaves so that the microwaves are only absorbed at a very low rate, or not absorbed at all. When a frozen product is reheated in a regular microwave transparent container the microwave energy is not properly absorbed by the frozen mass. Instead the major portion of the energy is concentrated at the interface region where the container contacts the frozen product. This uneven energy distribution is not equalized by convection heat transfer and results in excessive heating at the edges of the container with the core of the frozen mass remaining at a very low temperature. The microwave heating pattern of a large frozen dish is generally characterized by the presence of large cold spots in the center of the upper surface, by a very late thawing of the inner parts of the product, and by overheating at the edges and corners of the product.
EP 348 156 to Hewitt relates to an improvement in microwave heating wherein a microwave mode is generated from underneath the food product. The food product is disposed in a tray that is transparent to microwaves and the tray is placed on a stand so that a predetermined elevation is maintained between the bottom surface of the food product and the internal bottom surface of the stand. Heating from underneath occurs by placing separated electrically conductive plates at the bottom of the stand which are made of a microwave transparent material, or by making apertures in the electrically conductive bottom of the supporting stand. The purpose is to have a majority of the microwave energy enter through the undersurface of the container and maximize the bottom heating effect.
EP 185 488 to Sugisawa discloses a container, made of a material that is transparent to microwaves for use in a microwave oven. The container has a microwave reflecting strip that partly covers the region of the container where the upper surface of the material contacts the side of the container to prevent local over-heating of the food product. The container, however, brings no significant improvements in reheating of frozen foods and simply proposes a solution to the problem of local burning at the edges of the product when the product is reheated in a conventional transparent container.
EP 471 969 to Payne relates to the use of a microwave susceptor sleeve for pizza and the like onto which the food items are placed. The susceptor, with the food product on it, is placed on a supporting base. The supporting base is elevated with respect to the bottom of the microwave oven by the use of pre-cut legs. The elevation of the base supporting the susceptor is dictated by the need to separate the susceptor from the bottom of the microwave oven sole (i.e., the bottom surface of the oven cavity) to eliminate the risk of arcing when the oven does not have a glass shelf.
WO 93 23971 A to the Campbell Soup Company relates to a microwave metallic container wherein the bottom and the whole lateral walls are externally insulated using a polymeric or glass thin layer that completely isolates the container from the metallic parts of the microwave oven. The main features of the container are that it prevents arcing by insulating the aluminum inner tray. For better convenience and for a better heat distribution within foodstuffs that do not retain their initial shape, such as liquid foodstuffs, the bottom of the aluminum container may be slightly raised or domed so that the thickness of the product in the center of the container is reduced, since it is predominately the center of the product that has a cold spot upon microwave heating. Variations in the thickness of foodstuffs are, however, generally undesirable as it might create problems when removing the foodstuff from the container. In particular, the center of the foodstuff becomes more fragile than the periphery and this may lead to portions of the foodstuff breaking off when the foodstuff is removed from the tray. The slanted bottom of the tray also results in a more acute angle between the bottom and the sidewalls of the tray that further renders it more difficult to remove the foodstuff from the tray. Finally although the thickness of the foodstuff to be heated is reduced in the center part of the tray, the slanted bottom portion of the domed tray has a tendency to reflect the microwaves in an upward diverging direction and away from the center which causes a reduction in the microwave absorption in the center part of the foodstuff, and consequently cold spots in the center part of the foodstuff.
U.S. Pat. No. 5,310,980 to Beckett discloses the incorporation of metallic patches on a microwave transparent tray in order to orient the impinging microwave energy beams selectively towards parts of the product that do not heat-up appropriately.
EP 350 660 A2 to Jaeger relates to a susceptor sheet with a microwave transparent packaging.
U.S. Pat. No. 4,642,434 to Cox et al. relates to a microwave reflecting energy concentrating spacer that includes in its lower part a microwave reflector separated from the food base by a distance of about xc2xcof a wave length, i.e., about 3 cm, since the free space wave length at the microwave emitted frequency in the microwave oven (2.45 GHz) is about 12 cm.
EP 242 026 A2 to Swiontek discloses an assembly between a susceptor which is described as a xe2x80x9cmicrowave interactive layerxe2x80x9d and the whole package.
U.S. Pat. No. 4,656,325 to Keefer refers to xe2x80x9ccold susceptorsxe2x80x9d by placing metallic patches disposed in a regular array on the cover of a pan containing the food product.
U.S. Pat. No. 4,888,459 to Keefer also refers to xe2x80x9ccold susceptorsxe2x80x9d in addition to optimizing the thickness and the dielectric permittivity of the material constituting the non-reflecting part.
U.S. Pat. No. 5,270,502 to Brown et al. relates to a combination of a microwave interactive layer that is in fact a susceptor and a supporting stand made of a microwave transparent material.
WO 95 24 110 to Gics relates to an ovenable food package comprising a microwave susceptor placed beneath the food base in order to induce crispiness in the food base.
U.S. Pat. No. 4,496,815 to Jorgensen relates to a microwave browning utensil comprising a metallic base with a ferrite layer that is a highly microwave absorbing material.
U.S. Pat. No. 4,542,271 to Tanonis et al. relates to a microwave tray comprising an absorbing material placed beneath the bottom surface of the tray.
U.S. Pat. No. 4,927,991 to Wendt et al. relates to a microwave oven package comprising a combination of a grid and susceptors inside a microwave-transparent tray that behaves like a conventional frying pan as it is heated by microwave radiation that passes through the tray.
EP-A-0 451 530 to Peleg proposes to combine a susceptor sheet and a layer of heat absorbing material to control the heat flux to the bottom surface of the food product that is placed on the arrangement.
GB 2 226 220 to Mason discloses to a microwave-transparent tray with a microwave-transparent planar insert that raises the food product with respect to the tray bottom so that excess water and the fat from the food product may be collected into the base of the container below the supporting board.
U.S. Pat. No. 5,151,568 to Rippley is similar to the previous document with a corrugated wall placed on the bottom of the tray, instead of a planar insert. An absorbing material may be placed underneath the corrugated wall. Both the container and the corrugated wall are made of stiff paperboard material that is transparent to microwaves. The apertures in the corrugated allow liquids released by the food product during the heating to drain.
U.S. Pat. No. 5,041,295 to Perry et al. discloses a device made of a susceptor sheet and a thermal insulating pad or a rigid supporting layer so that the susceptor is thermally insulated from the bottom surface of the microwave oven.
WO 92 03355 to Guillot relates to a packaging device made completely of plastic having the general form of a container with a bottom and a sidewall. The plastic container and its lid are assembled in a snap-fitting arrangement.
U.S. Pat. No. 4,661,672 to Nakanaga discloses a container made of microwave-transparent material, the bottom surface of which is maintained at a prescribed elevation with respect to the floor of the oven, and a metallic device which is placed on the upper part of the food product in the container to control the uniformity of heating by preventing the upper parts and the edges of the food product from overheating.
Other prior art documents on microwave packaging are U.S. Pat. Nos. 4,994,638 and 4,535,889.
There remains a need for a container to cook frozen food products in a microwave oven that promotes a uniform and efficient distribution of heat within the product and avoids temperature gradients within the product after it is heated. There is also a need for a container that accelerates the microwave reheating of frozen food products, in particular, large blocks of frozen food.
The present invention is directed to a container for cooking food in a microwave oven. The container includes a tray having a bottom wall and a continuous side wall attached to the bottom wall and extending upwardly from the bottom wall, wherein the bottom wall and side wall define an interior cavity. The container also includes a support means made of a material that is substantially transparent to microwave radiation to support a food having a top surface and a bottom surface in the interior cavity. The food is supported in an elevated position above the bottom wall so that the bottom surface of the food is nearest to the bottom wall and the bottom surface of the food and the bottom wall are separated by a distance. The bottom wall of the tray, the bottom of the food, and at least part of the side wall defines a free space underneath the food, wherein the bottom wall and at least that part of the side wall that defines the free space are constructed to reflect substantially all microwave radiation that passes through the food back toward the food.
The side wall can extend upwardly above the free space at least to the upper surface of the food and is constructed to reflect microwave radiation to more quickly and uniformly heat the food.
The container may have a lid. The lid may be made of a material that is transparent to microwave radiation. At least a portion of the lid may be adapted to serve as the support means after the container has been opened. The lid may include a flat member and a peripheral edge that extends downwardly from the flat member and fits inside the interior cavity with the peripheral edge contacting the bottom wall of the container. The lid may include a flat member, a peripheral edge that extends downwardly from the flat member so that it forms a cavity for receiving the food when it is turned upside down, and a side edge associated with the peripheral edge and protruding outwardly from the cavity so that the side edge abuts with and is supported by the side wall of the tray and the flat member of the lid and the bottom wall are separated by a distance to provide the free space when the lid is turned upside down and placed in the tray.
The distance between the bottom surface of the food and the bottom wall may be at least about 2 millimeters but less than about 20 mm. The bottom wall and side wall part that defines the free space may constructed of a metallic material such as aluminum or an aluminum alloy. The tray may be a single unit made entirely of a metallic material that reflects microwave radiation. The tray may also be in a multilayer arrangement with at least one layer being a metallic layer that reflects microwave radiation.
The support means may be a flat member for receiving the food and at least one spacing member between the bottom wall and the flat member. In one embodiment there are a plurality of spacing members which are substantially evenly distributed and attached to the flat member. In another embodiment the at least one spacing member may be attached to the tray. The spacing member may also be a peripheral shoulder on the side wall directed toward the interior cavity onto which the flat member is positioned or a plurality of glass or plastic marbles that directly contacts the food. The support means may be collapsible and may be an inflatable bag having a series of air cells that define interior channels that can be inflated with air before cooking.
The side wall of the container may have one or more corners that are angled and further include a microwave opaque material that spans the side walls at the corners to concentrate the microwave radiation in the corners. The container may have a lid to provide the opaque material wherein the lid has a main central portion made of a material that is transparent to microwave radiation and microwave opaque portions are attached thereto. The container may have a cavity that is at least about 1 liter, and wherein the entire sidewall is constructed to reflect microwave radiation.
In one embodiment of the container the tray is a lid that in one position covers the support means and the food and in an inverted position forms the free space beneath the food. The support means may include a side wall to form the interior cavity and be configured to have a smaller periphery that than the side wall of the lid, and include spacer means for limiting the depth that the support means can be inserted into the lid. The spacer means may be a shoulder associated with one of the side walls.
The invention is also directed to a method for uniform microwave heating of food. The method involves providing a food having a top surface and a bottom surface, placing the food on a support means that is made of a material that is substantially transparent to microwave radiation, providing a free space beneath the bottom surface of the food wherein the bottom surface of the food wherein the bottom surface of the food is separated from a bottom surface of the free space by a distance and the free space is constructed to reflect substantially all microwave radiation that passes through the food back toward the food, and exposing the food to microwave radiation for a sufficient time to cook the food. The food product may be a frozen food product. The distance between the bottom surface of the food and the bottom surface of the free space may be between about 2 and 20 mm and the thickness of the frozen food product may be between about 28 and 60 mm. The support means may be surrounded by a side wall that has corners and the method may further include providing a microwave opaque material that spans the corners of the side wall to concentrate the microwave radiation in the corners.